Add a replica to a Percona Cluster

This feature was disabled as part of initial clustering to the Percona
datastore so this is here to enable it.

While Percona Cluster is a synchronous replication technology, it is
still imperative that asynchronous replication also be supported. To
this end, the following implementation details will be verified:

- The replica_of option of trove-create will support creating a
  standard percona instance as a replica of a specific instance in a
  percona cluster. This can potentially cause errors with replication
  using GTIDs.
  https://www.percona.com/blog/2015/02/13/percona-xtradb-cluster-5-6-a-tale-of-2-mysql-gtids/
- The replica_of option may be specified on the cluster-grow action
  to have an instance in the cluster receive asynchronous updates from
  another cluster or instance.
- The detach-replica command will sever the link between a replica and
  a replica source, even if either the replica or its replica source
  is a cluster.
- When a cluster is acting as a replica source, the
  eject-replica-source command will select a different node from the
  cluster rather than promoting a slave to be the new replica source.
- The promote-to-replica-source command will fail when either the
  replica selected or its replica source is a cluster.